A major objective of the US Energy Program is the development of processes to produce clean fuels from coal. Underground coal gasification is one of the most promising of these processes. If successful, UCG would quadruple the proven US reserves of coal. Costs of fuels produced by UCG are projected to be 65 to 75% of those produced by conventional surface coal-conversion processes. Finally, UCG may possess environmental advantages since no mining is involved and little solid waste is produced at the surface.